1. Field of the Invention
The present invention relates generally to solar collectors, and more particularly to a tensioning device for a stretched membrane collector.
2. Description of the Prior Art
Recent developments in the solar collector include a trend toward manufacturing reflectors for concentrating solar collectors with thin flexible lightweight reflector materials. Examples of such lightweight reflector materials are thin metallic sheets of steel or aluminum which are frequently referred to as foils. Collectors manufactured from these materials are commonly referred to as stretched membrane solar collectors. Generally speaking, a solar collector consists of a reflector and a metal-constructed reflector support frame. The reflector is typically in a form of a mirror or plurality of mirror segments.
Individual solar collectors are frequently employed in an array to concentrate solar radiation severalfold by reflecting and focusing the solar radiation onto an absorber/receiver. Solar radiation is commonly known as sunlight and, generally speaking, concerns electromagnetic radiation emitted by the sun. The absorber/receiver which may be of a cavity-type is positioned at the focal area of the array so as to absorb maximum heat energy.
The focal area, broadly speaking, concerns a point or region to which the collector reflects all of the incident sun radiation. The solar energy flux received and absorbed by the receiver/absorber is usually carried away by a suitable heat transfer fluid to either operate a thermodynamic heat engine or to provide process heat. Solar flux generally means energy flux transmitted from the sun, which is in the form of electromagnetic radiation.
The trend toward producing lightweight solar collectors is dictated in part by a high manufacturing cost of mirrored glass/metal-type reflector collectors. This trend is also dictated in part by the heavyweight of mirrored glass/metal-type heliostat reflector panels and their support structures. A heliostat may be simply defined as a tracking mirror. To continue, the reflector panels are typically fabricated from thick heavy metal, glass and composite materials in order to meet strength and rigidity requirements imposed by the heliostat performance. Speaking more specifically, such strength and rigidity is frequently required in order to give the panel the capacity to withstand environmental loads without undergoing warping, buckling or fracturing which eventually could lead to failure, as well as being required to maintain optical accuracy. Examples of such environmental loads are gravity loads, wind loads, and ice/snow loads.
Unfortunately, the heavy deadweight load of the reflector and the reflector suport frame frequently produces stresses and deformations in the heliostat which undesirably add to the harmful stresses produced by environmental loads. Additionally, the use of heavy structural elements and metal materials to add sufficient strength and rigidity so that the heliostats can sustain such loads is one major reason for their high manufacturing cost.
In addressing the disadvantages associated with the heavyweight collectors by producing collectors which employ substantially thinner and lighter weight manufacturing materials, a problem has developed in fabricating lightweight reflector support frames which can safely withstand stresses due primarily to twisting moments normally produced during the tensioning operation. In the tensioning operation, the reflector membrane is typically tensioned to provide a desired reflector surface contour. Unfortunately, however, some of the devices heretofore employed to tension the reflector membrane tension it by loading the reflector support frame substantially eccentrically.
It will be noted that tensioning of the membrane is usually required in order to provide an adequate focal point or image of the sun at the cavity of the absorber/receiver. A tensioned surface reflector will have a focal length which is a function of the reflector elevation angle and surface tension. The characteristics of a tensioned surface with respect to the associated focal point are normally used to enhance collector performance by reducing the size of the image at the receiver and therefore the amount of energy spillover.
Additionally, a problem has developed in providing lightweight stretched collector with variable or adjustable focusing capabilities, such that the collector can be used to produce various concentration ratios to meet specific collector site requirements. Concentration ratios concern the ratio of the intensity of solar light impinging on the absorber to that of the solar light impinging on the collective surface of the collector. Notably, these ratios may be as small as one for no concentration to as high as several thousand.
To cope with the aforesaid problems, the reflector surfaces of some solar collectors have been designed by tensioning a sheet of aluminized Mylar over a plurality of elongated supporting members. The supporting members function to impart a caternary configuration to the aluminized sheet. A prior art patent relating to such a design is U.S. patent Ser. No. 4,173,397. Unfortunately, however, this prior art design as well as others have suffered from one or more shortcomings. For example, this earlier design is unduly complex, comprises a number of component parts, and its focus is not easily controllable.
Some prior art designs have stretched a sheet of aluminized Mylar over the top of a hollow cylinder and reduced the pressure therein between to provide a desired surface configuration. An example of this design is disclosed in U.S. patent Ser. No. 4,288,146. However, unfortunately, this design may result in a proneness to develop leaks, and eventual changes in the pressure within the cylinder leads to undesirable and irreversible degradation of the collector focus. It will be noted that the use of a vacuum pump to maintain the desired pressure has to some degree been partly helpful in reducing some aspects of the problem with pressure leakage. However, a vacuum pump is an additional cost element and is power consuming.
Some prior art designs use flat surface-type collectors. In flat surface-type collectors, the reflected sun radiation is aimed rather than focused at the absorber/receiver cavity. Flat surface-type collectors, however, when employed in applications where high intensity ratios are desired, often produce an unacceptably enlarged focal region at the receiver as a consequence of a spreading of the reflected incident sunlight beam, as well as producing a related unwanted drop in optical efficiency. Optical efficiency generally concerns a measurement of a fraction of the sun energy that actually reaches the absorber/receiver cavity.